Scale deposits frequently occur in the production of water, oil and gas from subterranean formations and can result in plugged well bores, plugged well casing perforations, plugged tubing strings, stuck downhole safety valves as well as other valves, stuck downhole pumps and other downhole and surface equipment and lines, scaled formations and fractures in the vicinity of the well. Scale formation can occur as a result of mixing incompatible waters in the well which produce precipitates, or as a result of temperature and pressure changes in the produced waters during production. Generally, incompatible waters occur in waterflooding, as injected sea water mixes with formation water in the borehole during water breakthrough. The more common concern is scale deposited due to changes in supersaturation or solubility of minerals in the formation or produced waters caused by pressure and temperature changes, or changes in other physical and chemical parameters, such as gas compositions, ratio of gas/oil/water. Scale may also be formed from corrosion of metal equipment used in the subterranean oil and gas production. Scale formation is also a problem in aqueous systems used in cooling towers, boilers and the like. Precipitation frequently encountered as scale includes calcium carbonate, calcium sulfate, barium sulfate, magnesium carbonate, magnesium sulfate, and strontium sulfate.
Scale formation can be reduced by the introduction of inhibitors into the formation. Various inhibitors are known, including carboxylated polymers, organophosphates, organophosphonates and polyphosphonates. Typically, carboxylated polymers are polymers and copolymers of acrylic or methacrylic acids, commonly referred to as polyacrylic acids. Organophosphorous-containing inhibitors include alkyl ethoxylated phosphates; ethylenediaminetetramethylene phosphonic acid; aminotrimethylene phosphonic acid; hexamethylenediaminetetramethylene phosphonic acid; diethylenetriaminepentamethylene phosphonic acid; hydroxyethylidenediphosphonic acid and polyvinyl phosphonic acid. The injection of scale inhibitors without pre or post cross-linking to protect an oil or gas well from mineral scale formation is widely practiced. However, such treatments often result in poor retention in the subterranean formation, quick depletion and frequent retreatments.
In an article that appeared in the Journal of Petroleum Technology on page 1303 in June 1982, A. J. Essel and B. L. Carlberg disclosed a method to increase retention of an inhibitor in a subterranean limestone formation by injecting the acid form of a polyphosphonate inhibitor which forms a slightly soluble calcium salt. Calcium ions released on dissolution of some of the limestone rock by the acid precipitates calcium polyphosphonate allowing greater retention in the rock. However, this method does not work in sandstones, because sandstones are not soluble in acids, nor do they form calcium ions even when dissolved.
U.S. Pat. No. 3,827,977 issued to L. A. Miles et al discloses a method of increasing inhibitor retention by in situ formation of the relatively water or brine insoluble polycation salt of polyacrylic acid or hydrolyzed polyacrylamide. A disadvantage in this method is that a large excess of metal ion is required which increases the danger of damaging the formation from premature precipitation.
Therefore, what is needed is a method to increase the retention time of an inhibitor which method will not damage a formation and which can be used in substantially all formations.